I have a question about gain setting of LTC6419 ADC driver.
Can we use LTC6419 by AV=0.5[V/V]? If yes, are there any recommend about the value of RI RF and CF?
You may be better off putting a 6dB pad in front of the LTC6419 (configured for 0dB gain) instead. The LTC6419 guarantees stability for 0dB gain but no mention of any gain lower than that.
Here is an LTC6419 0dB gain setup as an example (open with LTspice):